cachefiles: enable on-demand read mode

Enable on-demand read mode by adding an optional parameter to the "bind"
command.

On-demand mode will be turned on when this parameter is "ondemand", i.e.
"bind ondemand". Otherwise cachefiles will work in the original mode.

Signed-off-by: Jeffle Xu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Acked-by: David Howells <[e-mail]>
Signed-off-by: Gao Xiang <[e-mail]>

fs/cachefiles/daemon.c

···
 	    cache->brun_percent  >= 100)
 		return -ERANGE;
 
-	if (*args) {
-		pr_err("'bind' command doesn't take an argument\n");
-		return -EINVAL;
-	}
-
 	if (!cache->rootdirname) {
 		pr_err("No cache directory specified\n");
 		return -EINVAL;
···
 	if (test_bit(CACHEFILES_READY, &cache->flags)) {
 		pr_err("Cache already bound\n");
 		return -EBUSY;
+	}
+
+	if (IS_ENABLED(CONFIG_CACHEFILES_ONDEMAND)) {
+		if (!strcmp(args, "ondemand")) {
+			set_bit(CACHEFILES_ONDEMAND_MODE, &cache->flags);
+		} else if (*args) {
+			pr_err("Invalid argument to the 'bind' command\n");
+			return -EINVAL;
+		}
+	} else if (*args) {
+		pr_err("'bind' command doesn't take an argument\n");
+		return -EINVAL;
 	}
 
 	/* Make sure we have copies of the tag string */